SYNAPTIC PLASTICITY OF THE INTERPOSITO-RUBRAL PATHWAY THAT IS FUNCTIONALLY
RELATED TO FORELIMB FLEXION MOVEMENTS.
PANANCEAU, MARC, LUCIE RISPAL-PADEL AND EL MEHDI MEFTAH.
Centre de Recherche "Cerveau et Cognition", UMR 9940, Centre National de la
Recherche Scientifique, Facult[acute]e de M[acute]edecine de Toulouse
Rangueil, 31062 Toulouse, France.APStracts 3:0014N, 1996.
SUMMARY AND CONCLUSIONS
1- Some connections from the afferents to the magnocellular red nucleus (RNm),
like the cortico-rubral synapses, have plastic properties which are thought to
contribute to long-term changes such as functional re-adaptation, motor
learning and the establishment of conditioned responses. Since previous
studies had focused on cortico-rubral synaptic reorganization after these
events, we attempted to investigate cerebello-rubral connections in intact
adult cats during associative conditioning by pairing electrical stimulation
of interpositus nucleus (the CS) with electrical simulation of the forelimb
(the UCS). A large increase in the amplitude of the forelimb flexion (CR) to
the CS was observed after several days of paired CS-UCS presentations. 2- For
this purpose, both behavioral and electrophysiological methods were used to
correlate synaptic plasticity with changes in the motor responses. The
somatotopically organized sensorimotor network functionally related to the
control of the elbow joint movements was studied in awake adult cats. This
circuit was defined on the basis of sites at which elbow flexions could be
evoked both as a conditioned (CS) and unconditioned (UCS) stimulus. The CS was
applied in the cerebellar interpositus nucleus site (IN) and the UCS was given
to the skin on the dorsum of the distal part of the forepaw. Daily classical
conditioning consisted of repetitive pairings of CS and UCS with an
interstimulus interval (ISI) of 100ms. 3- The transmission efficacy resulting
from the conditioning was tested in various targets of the cerebellar efferent
pathway, including the RNm. Electrophysiological responses evoked in these
relay structures by the CS and the forelimb angular deviations were
simultaneously recorded throughout each daily conditioning session. The
surface areas of the rubral responses to CS and the percentage response rate,
the angular deviation (amplitude) and latency of the motor responses were
systematically measured throughout the conditioning procedure. Test sessions
were also performed before and after each period of conditioning.
Quantification and statistical analysis were carried out to determine whether
changes observed in interposito-rubral synaptic transmission and in the motor
responses evoked by the CS were correlated. 4- Daily repetition of paired CS
and UCS according to a predefined and fixed temporal schedule led to an
increase in the response rate and amplitude of the forelimb flexions. A
procedure with repeated presentation of CS preceded by UCS was used to produce
extinction of the enhanced motor responses. The associative nature of these
changes was confirmed by the fact that the CS given alone for eleven days in a
control condition failed to produce any modification of the motor response. 5-
The changes in the flexion movements were accompanied by a nearly parallel
increase of the amplitude of the "post-synaptic field potentials" evoked in
the RNm by the CS (interpositus nucleus stimulation). Changes in the
transmission efficacy of the interposito-rubral synapses stayed stable even
after several days of interruption and remained constant up the extinction
period. Changes affecting both the motor and the central responses were
significantly correlated suggesting that modifications in the interposito-
rubral transmission efficacy might be one of the plastic correlates of
forelimb flexion conditioning. 6- Examination of the neuronal excitability
within either the interpositus nucleus or the RNm or in the spinal cord failed
to show any evidence of facilitation suggesting that the increases in the
post-synaptic rubral field potential were attributable to a plasticity of the
interposito-rubral connections. The long lasting duration of the increase of
cerebello-rubral synaptic transmission suggests that structural changes were
induced by conditioning in the intact animal. These changes could either take
part in the refinement of the pathways involved in the newly learned movements
or mediate the adaptation of the motor command in response to the external
constraints. Thus, the "activity-dependent" synaptic plasticity could be one
of the neuronal correlates of behavioral adaptation processes but it could
also indicate the RNm participation in some aspects of motor learning.

Received 7 July 1995; accepted in final form 28 December 1995.
APS Manuscript Number J431-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 25 January 96